Inkjet printers operate by ejecting small droplets of ink from individual orifices in an array of such orifices provided on a nozzle plate of a printhead. The printhead forms part of a print cartridge which can be moved relative to a sheet of paper and the timed ejection of droplets from particular orifices as the printhead and paper are relatively moved enables characters, images and other graphical material to be printed on the paper.
A typical conventional printhead is fabricated from a silicon substrate having thin film resistors and associated circuitry deposited on a front surface of the substrate. The resistors are arranged in an array relative to one or more ink supply slots in the substrate, and a barrier material is formed on the substrate around the resistors to isolate each resistor inside a thermal ejection chamber. The barrier material is shaped both to form the thermal ejection chambers, and to provide fluid communication between the chambers and the ink supply slot. In this way, the thermal ejection chambers are filled by capillary action with ink from the ink supply slot, which itself is supplied with ink from an ink reservoir in the print cartridge of which the printhead forms part.
The composite assembly described above is typically capped by a metallic nozzle plate having an array of drilled orifices which correspond to and overlie the ejection chambers. The printhead is thus sealed by the nozzle plate, with the only path for ink flow from the print cartridge being via the orifices in the nozzle plate.
The printhead operates under the control of printer control circuitry which is configured to energise individual resistors according to the desired pattern to be printed. When a resistor is energised it quickly heats up and superheats a small amount of the adjacent ink in the thermal ejection chamber. The superheated volume of ink expands due to explosive evaporation and this causes a droplet of ink above the expanding superheated ink to be ejected from the chamber via the associated orifice in the nozzle plate.
Many variations on this basic construction will be well known to the skilled person. For example, a number of arrays of orifices and chambers may be provided on a given printhead, each array being in communication with a different coloured ink reservoir. The configurations of the ink supply slots, printed circuitry, barrier material and nozzle plate are open to many variations, as are the materials from which they are made and the manner of their manufacture.
Because of their very small dimensions, printheads of this general type have the disadvantage that the ink passageways of the structure are liable to blockage by ink particles or other contaminants. One way to avoid this is to provide alternative ink supply paths that bypass the main ink supply slot and provide alternate paths for ink—see, for example, our U.S. Pat. No. 6,364,466. FIG. 7 of that patent shows shallow ink bypass channels which extend laterally away from the main ink supply slot and allow ink to reach the ink ejection chambers even though the main ink supply slot is blocked.
However, such solutions tend to involve complex structures which lead to additional undesirable processing steps. They also provide small features which can trap bubbles of air entrained in the ink.
It is an object of the invention to provide a new construction of inkjet printhead in which these disadvantages are avoided or mitigated.